Lasers are well accepted as essential tools for ophthalmic treatments. A laser treatment device requires four basic elements in order to effectively deliver laser energy into the eye for medical treatment. The four essential elements are:                An illuminator delivering light to the target treatment area to assist viewing;        A microscope (preferable binocular) for viewing the target treatment area;        Optics to deliver aiming beams to the target treatment area to assist with accurate targeting of the treatment; and        Optics to deliver the treatment laser beam to the target treatment area.        
It is known to use a slit lamp to provide the illumination and to view the target treatment area with a binocular microscope. It is also known to use a dichroic mirror to reflect the treatment laser beam and aiming beams into the microscope viewing path. In order to deliver the illumination, aiming beams and treatment beams without blocking the binocular viewing path, it is usual to provide the illumination from a position outside the main optical path. The aiming beams, treatment laser beam and viewing path are focused by an objective lens. A typical prior art arrangement is shown in FIG. 1.
The arrangement in FIG. 1 is functional but does not provide illumination of the treatment area coaxially with the viewing or laser paths. This is mot usually a problem for anterior treatments of the eye since the illumination adequately illuminates the treatment area. For posterior treatment areas there is a problem since the optical path for the treatment laser beam, aiming beams, illumination and viewing must pass through the aperture created by the iris. A typical dilated iris is about 7.8 mm and is located about 20 mm from the retina. The angles are such that for treatment of the posterior of the eye virtually coaxial delivery of the treatment laser beam, aiming laser beam, illumination and viewing is required. This is not possible with the geometry of FIG. 1.
One approach to overcome this problem has been to use dual illumination mirrors placed between the binocular viewing paths (JP10328226) with a small spacing to allow laser beams to be delivered between them, thus providing almost coaxial illumination. The problem with this approach is that the illumination is not quite coaxial and the reduced aperture for the treatment laser beam restricts the laser beam diameter.
The restricted treatment laser beam diameter is a particular problem for short pulse photo-disruptor lasers that require a full cone angle of 14 to 18 degrees and a working distance of 90 to 100 mm. These parameters require the laser beam emitted from the microscope to have a diameter approximately the same diameter as the viewing objective lens, so that coaxial illumination with the existing arrangements is not possible without blocking a portion of the treatment laser beam.